COMING DOWN THE MOUNTAINS: INTERPRETATIONS OF TERRACE DEPOSITS AND DEBRIS FLOW SEDIMENTS IN CLEAR CREEK WATERSHED, WHISKEYTOWN NATIONAL RECREATION AREA, CALIFORNIA (Invited Presentation)

Post fire debris flows in California, such as the destructive 2018 Montecito event, are of increasing scientific interest. In Whiskeytown National Recreation Area (WHIS), located in the Trinity Range of northern California, the 2018 Carr Fire burned significant areas of steep terrain, but without triggering significant flow events. Ample evidence of pre-historic debris flows in the form of boulder levies and deposits are found in many drainages of WHIS. In addition, there have been historic destructive debris flows initiated at improperly abandon forest roads. This evidence of a debris-flow-prone terrain leads to questions about the future post-fire behavior of the area. Flows emanating from the Shasta Bally, the highest peak within WHIS, entrain a coarse quartz-rich decomposed granodiorite, called grus. Prior to the 1960s damming of Clear Creek, debris flows presumably could run from the upper slopes to the confluence of the Sacramento River. Below the dam, the creek still runs through a bedrock canyon. A ½ km reach, near the confluence with Paige-Boulder Creek, is bordered to one side by a ~4m high terrace, with a tread exceeding 100 m width. Sediments within the terrace are quartz rich and are presumably amenable to optically stimulated luminesce (OSL) age analysis. Excavations in May 2022 into the terrace reveal a massive >2m thick fine-grained sediment unit over grus-rich channel deposits and cobbles. We report new interpretations of sediment emplacement, landform stability and stratigraphy, with geochronologic control through OSL and ¹⁴C ages. Additionally, paleowildfire-regime data are derived from buried microscopic charcoal and facies analysis. One primary inference is that an unknown downstream flow constriction in Clear Creek, may have emplaced the finer-grained sediment under backwater conditions. Further research is needed to support this hypothesis since remote sensing data for the area does not show an obvious geomorphic signature for a former impoundment.